1. Field of the Invention
The invention relates generally to metal ore refining and specifically to the electrolytic reduction of alumina, aluminum sulfates and potassium aluminum sulphate into aluminum metal.
2. Description of the Prior Art
Vast deposits of oil exist throughout the world, and especially in Canada, as thick, heavy oil, in the form of bitumen mixed with solid minerals and water. The tar sands that hold the bitumen contain rich amounts of valuable minerals, especially alumina, as clay in the sand itself. The sands include a fines fraction, defined as particles less than forty-four microns, that have a clay component (0-20 microns) and a silica fine sand component (20-44 microns). High bitumen content in the tar sand is usually associated with a low clay content. Conversely, a low bitumen content in the tar sand is usually associated with a high clay content. This is because it is the sand, and not the clay, that embraces the bitumen.
Typically there are found two parts silica fine sand component to one part clay component, e.g., one-third is clay. About 35% of such clay is alumina. Certain low grade ores, conventionally comprised of undifferentiated silica fine sand and clay, have as little as 6% alumina. Such low-grade ores are a problem when used in exothermic reactions that separate out the alumina. High-grade ores, with more than 10% alumina, are much more easily processed exothermically.
The principle commercial method used for the electrolytic reduction of alumina to aluminum is the Hall-Heroult process. This traditional process uses a molten bath of sodium cryolite (Na.sub.3 AlF.sub.6) that is contained in a cell lined with carbon. A pool of molten aluminum lies at the bottom of the cell and serves as the cell's cathode. Consumable carbon anodes are dipped down into the electrolyte bath. Alumina is introduced to the bath which dissolves the alumina and aluminum reduction occurs in the form of liquid aluminum droplets. Typical operating temperatures are 950.degree. C. (1,742.degree. F.) to 1,000.degree. C. (1,832.degree. F.). Carbon dioxide is released, from a reaction of the oxygen electrically-forced from the alumina with the carbon in the anodes. As such, the carbon anodes are consumed and must be periodically adjusted and/or replaced. Large amounts of electricity are also required, which makes aluminum recycling a competitive source of aluminum metal.
On Jun. 3, 1986, U.S. Pat. No. 4,592,812, was issued to Theodore R. Beck, et al., which describes the electrolytic reduction of alumina. A cell used in the reduction has an electrolyte bath of halide salts. A non-consumable anode is positioned at the bottom of the bath, and a dimensionally-stable cathode coated with titanium diboride is spaced above in the bath. Particles of alumina are introduced to the bath and form ions of aluminum and oxygen. The oxygen ions are converted to gaseous oxygen at the anode when electricity is applied. The gaseous oxygen bubbles at the anode and agitates the bath. The aluminum ions are converted to metallic aluminum at the cathode. The cell temperature is just high enough to keep the metallic aluminum molten, and the liquid aluminum accumulates as a pool on top of sludge at the bottom the bath and the secondary cathode.
Theodore R. Beck, et al., were issued U.S. Pat. No. 4,865,701, on Sep. 12, 1989, which describes another electrolytic cell with a bath of halide salts. The anodes and cathodes are vertical plates that are interdigitated and dipped from above into the bath. Bubbling of oxygen at the anodes agitates the bath and resists the settling of alumina particles at the bottom of the bath. Molten aluminum droplets form at the cathodes and flow down to accumulate at the bottom of the bath in a sump.
The use of finely-divided alumina particles in the electrolytic reduction of alumina to aluminum is described by Theodore R. Beck, et al., in U.S. Pat. No. 5,006,209, issued Apr. 9, 1991. Alternating, vertically-disposed cathodes and anodes are used with a horizontally-disposed gas-bubble generator in a molten electrolyte bath of balanced amounts of NaF+AlF.sub.3 eutectic, KF+AlF.sub.3 eutectic and LiF. The gas-bubble generator keeps the alumina particles in suspension. The bath eutectics allow the cell to be operated at a substantially lower temperature, e.g., 660.degree. C. (1220.degree. F.) to 800.degree. C. (1472.degree. F.). The cathodes are made of titanium diboride (TiB.sub.2), a refractory hard metal. The anodes are composed of nickel-iron-copper (Ni-Fe-Cu) cermet. The mean size of the alumina particles introduced to the bath ranges between one micron and one hundred microns, preferably within a range of two to ten microns. The smaller alumina particle sizes are described as being easier to maintain in suspension. But such fine particles are said to have a tendency to agglomerate into clumps which settle out of the bath rapidly. So bottom-located gas generators in the bath are included to deal with this problem.
Theodore R. Beck, et al., describe a non-consumable anode and lining for an aluminum electrolytic reduction cell in U.S. Pat. No. 5,284,562, issued Feb. 8, 1994. The electrolyte used has a eutectic of AlF.sub.3 and either NaF, or primarily NaF with KF and LiF. The anodes used are made of copper, nickel and iron.
A cell for the "production of aluminum with low-temperature fluoride melts" is described, by Theodore R. Beck, in Proceedings of the NS Light Metals Committee, from the 123rd TMS Annual Meeting in San Francisco, Calif., Feb. 27, 1994 to Mar. 3, 1994, pp. 417-423, as published by The Minerals, Metals & Materials Society (TMS) 1994. The proposed commercial cell design uses a eutectic electrolyte with a freezing point below 695.degree. C. of either NaF with AlF.sub.3 or a mixture of NaF/AlF.sub.3, KF/AlF.sub.3 and LiF/AlF.sub.3, eutectics operating about 750.degree. C. A 5-10% slurry, by weight, of Al.sub.2 O.sub.3 with a particle size less than ten microns is required. Close-spaced vertical monopolar anodes and TiB.sub.2 cathodes are used, which makes a potroom to house a potline of such cells dramatically reduced in size over the conventional horizontal-cell potrooms. A horizontal bottom auxiliary anode is used in the cell to agitate the electrolyte to keep sludge from forming from alumina that falls out of suspension, as occurs when the alumina particles agglomerate or are individually larger than ten microns. A device to continuously transport out aluminum produced by the cell is identified as a necessity, but no suitable mechanism is described. Also, feedstocks of alumina with particle sizes less than forty-four microns are generally not available, e.g., because of the severe dust problem such powders can produce. Alumina is injected into the bath from above and contributes to a dust problem due to oxygen capturing alumina dust as it leaves the molten electrolyte surface. In addition, it is difficult in the envisaged tall cells to insure that the alumina reaches all the areas of electrolysis. This and the separation of the aluminum from the bottom sludge are problems for the commercial operation with unspecified solutions. Therefore, the description here by Beck of a practical commercial cell is incomplete.